1. Field of the Invention
The present invention relates to a method of dwelling in injection molding carried out by using both an injection machine having a built-in injection screw and a dwelling machine having a built-in plunger.
2. Background Art
It is well known in the art to perform injection molding by using both an injection machine and a dwelling machine. The injection machine for this purpose is connected to the dwelling machine. More specifically, the injection machine has a heating cylinder where an injection screw is assembled in a rotatable and slidable manner. The injection machine is connected to a nozzle member through a resin passage. The resin passage is opened and closed by a valve. The dwelling machine has a hydraulic dwelling plunger slidably disposed in a cylinder at the end of the dwelling machine. The dwelling plunger faces to a portion of the resin passage downstream from the valve. For a dwelling cycle, the head of the plunger may be advanced into the resin passage to press the resin. Alternatively, a portion of the resin in the resin passage may be forced into the cylinder by means of retracting or withdrawing the plunger to its original position. This resin is then forced beck to the resin passage by means of advancing the dwelling plunger after injection of the resin into a mold is completed.
The above mentioned conventional methods tend to be affected by a negative pressure. This is mainly because the forced retraction or withdrawal of the dwelling plunger upon completion of the dwelling cycle. The amount of the pressure applied to the resin greatly relates to a backward stroke or a distance of backward movement of the dwelling plunger. A relatively large amount of resin is thus forced into the cylinder when the backward stroke used is relatively large for a higher pressure to the resin upon dwelling. The negative pressure may be a cause of defective molded article with voids, air bubbles, or silver. In addition, it is difficult to reduce the time for injection because of a limited amount of compression.
The present invention is directed to overcome the above mentioned problems in the conventional dwelling method and, an object thereof is to provide a novel method of dwelling in the injection molding in which a resin for use in dwelling is stored on a side of the resin passage without forcefully retracting the dwelling plunger after the dwelling cycle, thereby to avoid generation of air bubbles and silver and with which the stored resin is forced to solve a problem of unstable dwelling due to a change in volume.